1. Field of the Invention
The present invention is directed to a system for locating the position of a unit in a communication network and to a method for performing such position location. More specifically, it is directed to a system and method for performing position location in a mobile communication network such as a cellular telephone or personal communication system (PCS) network.
2. Description of Related Art
The ever-increasing popularity of mobile communication devices such as cellular telephones and the like brings with it a need for increased reliability and functionality of those devices. As users come to rely on wireless telephone networks more and more and those telephones become ubiquitous in everyday life, the networks must be capable of accommodating an ever-widening range of demands.
One such demand, promulgated in June, 1996 by the U.S. Federal Communications Commission (FCC) as FCC Docket Number 94-102, requires that future wireless services provide a site location feature for its mobile units. This feature is primarily for use in providing emergency call service capability, complementary to the emergency 911 (E-911) service familiar to most wireline telephone users. To provide emergency services to a caller, it is advantageous for any communication system used to make an emergency call to be able to automatically identify the location of the caller. This is because in emergency situations time is of the essence and further, the caller may not know his or her location, may give an incorrect or otherwise inaccurate location, or may become incapacitated during the course of the call.
In the case of wireline networks, such position identification is a relatively simple matter of correlating the caller's telephone number with a list of numbers and corresponding addresses. In the case of wireless E-911 service, however, the very mobility which makes the portable telephones so useful precludes a simple lookup technique for position location. Thus, a method of "dynamic" position location must be used in mobile networks.
A number of position location systems are based on the Global Positioning System (GPS) implemented by the U.S. Department of Defense (DoD). GPS is a constellation of twenty-four active satellites circling the earth in precisely timed and controlled orbits so that between five and eight satellites are theoretically in line-of-sight radio contact with any point on earth at a given time. Each satellite broadcasts a uniquely-coded signal which can be picked up by an appropriately equipped GPS receiver, or "rover". The rover timing is synchronized with that of the satellites and is equipped with ephemeral data that allows it to precisely calculate the position of each of the satellites at any given time. It receives signals from three or more of the satellites and calculates its distance from each based on the travel time of its respective signal. Each of the distances defines a sphere centered on its respective satellite, and the intersection of all spheres is the actual location of the rover.
GPS position location is not perfect, however. Various sources of errors can cause the rover's detected position to differ from its actual position significantly, and their combined effect may even be to prevent the system from meeting the minimum 125 meter accuracy requirement set by the FCC. In civilian applications, the largest error source by far is called "selective availability" and is the result of the DoD deliberately decreasing the integrity of the civilian portion of the GPS satellite signals to deny hostile parties high-accuracy positioning capability. Other errors are due to natural conditions, such as ionospheric and tropospheric conditions, and some, such as satellite clock and orbit errors, receiver noise and multipath propagation effects, stem from artificial sources.
Multipath propagation results when a radio wave travels from a source (e.g., a GPS satellite) to a destination (e.g., a rover) through a space populated with objects that reflect radio waves. As shown in FIG. 1, if a line-of-sight path exists between the satellite and rover, direct reception occurs via a true signal TS. However, the signal may be reflected off of objects in the region to produce reflected waves which travel a longer distance and therefore are delayed relative to the original signal and appear as separate signals to the rover. Multipath reflection signals may include components from two sources: static components which result from reflections off of stationary objects such as mountains, buildings and the like, such as signal SR; and dynamic components which result from reflections off of moving objects such as vehicles, such as signal DR.
FIGS. 2A-2C show the effects of multipath propagation on the position location process. FIG. 2A shows an idealized typical topography, e.g., a city area of several blocks. FIG. 2B shows the same area as it might appear to rovers in the area based on GPS position information distorted by multipath propagation. FIG. 2C shows how the position of a rover in the area might be mistakenly detected based on such corrupted information.
Although the deleterious effects of multipath propagation have been illustrated in the context of GPS position location, such problems will of course be encountered in many types of communication with a rover, e.g., communication from a base station or the like.
With the exception of multipath propagation, all of these errors are experienced by receivers in the same general area to an equal degree, and a technique called "differential GPS" (DGPS) makes use of this fact to increase position location accuracy. DGPS uses a reference receiver whose actual position is precisely known in proximity to the mobile receiver. The reference receiver calculates its predicted position based on signals from the GPS satellites and compares it to its actual, precisely known position to derive a differential correction. This information is passed along to the rover via a separate information channel, e.g., radio, and is used by the rover to correct its position. Multipath propagation effects cannot be corrected in this way, since they change significantly with a small change in position and a particular set of corrections are generally valid only for a given rover at a given location at a given moment in time.
It is possible to incorporate a rover into a mobile telephone unit and to provide the GPS positional information during an E-911 session; however, this approach has some drawbacks. For example, GPS receivers are relatively expensive, and including such a unit in a cellular telephone or the like would substantially increase its cost (particularly one with DPGS capability). Further, GPS receivers are complicated electronic devices which consume significant amounts of power. Inclusion of one in a portable telephone would either shorten the telephone's battery life or require the use of larger capacity (and consequentially larger size) batteries. Thus, the telephone's size must be increased to accommodate the batteries and the additional GPS circuitry.
Moreover, GPS satellites provide a relatively weak, high frequency (carriers in the range of 1.2-1.6 GHz) signal which does not penetrate buildings and other dense structures well and which requires the use of specialized directional antennas. Additionally, DPGS operation requires the use of reference receivers which are not universally available. Further, the first position determination by a GPS receiver after it is turned on (called a "cold reading", as opposed to a "hot reading" made by a unit that has been operating for a period of time) can take up to fifteen minutes--clearly unacceptable in the E-911 environment. Finally, the need for a GPS receiver in each handset would mean that existing handsets lacking such functionality could not be used for wireless E-911 position determination.
Some of these problems are solved by a technique called "cellular geolocation", in which cellular base stations monitor transmissions from a mobile unit (typically, reverse voice channel or reverse control channel transmissions) and apply an angle of arrival (AOA) or time difference of arrival (TDOA) technique to the transmissions to determine the position of the unit. As described in Rappaport et al., "Position Location Using Wireless Communications on Highways of the Future", IEEE Communications Magazine pp. 33-41 (Oct. 1996), the AOA technique uses highly directional antennas to determine the precise angle at which each base station receives the mobile unit transmission, and the mobile unit's position is resolved by triangulation with the known positions of the base stations. Much like GPS, the complementary TDOA technique measures the relative delay in reception of the mobile transmissions at each base station, determines the distance traveled by the transmissions to each base station based on its respective delay, and resolves the mobile unit's position by resection.
These approaches, however, are still susceptible to position determination errors resulting from multipath propagation and other effects. The need to take such error sources into account cannot be overlooked in E-911 applications, and is particularly critical in congested urban areas where users are in close quarters and concentrated multipath environments exist.